The present invention relates generally to a magnetic resonance (MR) imaging and, more particularly, to a system and method for real-time, un-gated localization of desired slices for gated MR imaging. The present invention is also capable of producing a prescribed fixed number of images for each R-R period of a cardiac cycle.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, or “longitudinal magnetization”, MZ, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B1 is terminated and this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx, Gy, and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
Magnetic resonance imaging is a diagnostic imaging technique commonly used to review, identify, and diagnose pathologies or abnormalities in a scan subject, e.g., medical patient. In particular, MR images of the cardiac region are often used by health care professionals to aid in ventricular assessment. Traditional MR evaluation of ventricular functions often rely on repeated cardiac-gated acquisition of MR data in order to reduce image degradation resulting from the continuous movement of the cardiac region. These gated acquisitions are often performed over multiple cardiac cycles requiring a patient to undergo multiple breath-holds to acquire the requisite MR data to reconstruct the full set of images suitable for diagnosis.
While such cardiac-gated acquisition methods allow the cardiac region to be imaged, the images are susceptible to decreased image quality resulting from variances in breath-hold positions. Specifically, the images often include “misregistration” and may be of poor quality because a patient's breath-hold position differs across acquisition intervals. Furthermore, since cardiac patients suffer from conditions that make reliable cardiac-gated triggering difficult, such as irregular cardiac rhythms, the propensity for degraded images increases significantly with these patients. Additionally, requiring multiple breath-holds of a patient with respiratory problems may be tiring and can increase the sensitivity to differences in the breathhold position. As such, image quality can be affected when imaging a patient having respiratory ailments and/or an irregular cardiac rhythm, as is not uncommon among cardiac patients.
As a result, methods have been developed to reduce image artifacts and misregistration due to cardiac irregularities and/or breath-hold variances. For example, un-gated single-shot imaging techniques have been utilized to reduce misregistration by providing full heart coverage in a single breath-hold. Such methods reduce sensitivity to arrhythmias by acquiring MR data in real-time (i.e. without cardiac gating). However, the acquired data is not synchronized to the cardiac cycle of the patient and the number of acquired images varies from beat-to-beat and, as a result, from location-to-location.
Other, “triggered,” real-time MR processes have been developed and implemented to synchronize MR data acquired during real-time imaging to the cardiac cycle. These triggered real-time MR methods rely on automated trigger-based registration methods. Cardiac trigger signals are monitored during continuous real-time scanning and scan parameters are automatically modified in response to each beat within the cardiac cycle. As a result, the acquired data can be synchronized within the cardiac cycle. However, such methods can still produce a variable number of images for each R-R period, which is undesirable for clinical evaluations. That is, these systems are not capable of imaging a prescribed fixed number of cardiac phases for the slice during a given R-R interval. Moreover, many methods are not capable of imaging an entire slice within a single R-R interval of the cardiac cycle. For example, a portion of a first slice is typically imaged during a first R-R interval and then, during a second R-R interval, the remaining portion of the first slice is imaged. As a result, blurring and/or artifacts may result in the reconstructed image. Furthermore, such methods also become sensitive to arrythmias.
It would therefore be desirable to have a system and method capable of reducing the potential for misregistration as well as breath-hold requirements in cardiac imaging. It would also be desirable to have an imaging technique that may be synchronized to the cardiac cycle and yield a prescribed fixed number of images per R-R period. Further, it would be advantageous to allow interactive localization and adjustment of slices and provide full heart coverage along both short and long axis orientations in a single breath-hold.